Food is considered to favor growth of unwanted cells such as bacteria, yeast, and fungi. In general non-pathogenic bacteria are considered as having no negative consequence for human health. Several are even known to have beneficial effects on our organism, for example L-caseT-Immunitas which facilitate the intestinal transit. However, several pathogenic bacteria such as Salmonella and Listeria are capable to grow fast in rich environments such as food. In the last decades serious health problems are frequently arising caused by these contaminating cells. Therefore many efforts have been made to find tools to detect these contaminations before the food is distributed on the market.
In general, strategies for the characterisation of bacterial and/or other contamination consists of a two-step method:                the first step involves the amplification, i.e. a bacterial multiplication in a sample of the food suspension to be analyzed,        the second step involves the detection, i.e. the proof of the presence of pathogenic bacteria.        
The total absence or the minor presence (a non-toxic dosis) of the pathogenic bacteria will allow a safe consumption of the respective food.
If the amplification is done in a rich culture medium all bacteria or cells present in the sample will grow. This step is not specific and may result in a severe miscalculation of the pathogenic bacteria. Indeed, a non-selective medium will amplify all the cells in the sample, including the target cells, and the subsequent detection will be much more laborious and less reliable.
In order to solve this known problem it is of importance to have an amplification which is performed selectively resulting in the enrichment of the cell which needs to be detected in the second step. Consequently, methods have been developed using a specific medium comprising specific growth factors allowing the growth of only the cell which one need to detected lateron. This is performed by optimising culture conditions to favor the target cells growth and disfavor all the other cell growths. For each cell type a specific temperature, medium and time of incubation is chosen. In general, the required culture time will be short if the medium is rich but allowing all present cells to grow. Unfortunately, these methods need an extensive input to optimize time of incubation and selectivity of the medium wherein the target cell grows faster compared to the growth of background cells. Some selective media have been developed by optimizing their composition but they are usually poor selective and/or poor nutritive. In the latter case, the time of incubation need to be extended.
However, these methods have a serious inconvenience: by increasing the specificity of a medium the nutritional elements will be provided in at much lower concentration resulting in a considerable decrease of the growth rate of the bacteria in this medium. Indeed, samples which are taken from food for rapid consumption need to be evaluated quickly for the presence of pathogenic organisms. Because enterprises can not allow a slow process for analysis one need a specific and fast identification method for contaminating and/or pathogenic cells. In addition, the extensive investment to select, determine such a medium and the time needed for cultivation is a major drawback of this approach.
There is a lot of literature concerning selective enrichment media used for the detection of pathogenic bacteria in food (see de Boer E: Update on media for isolation of enterobacteriaceae from foods. International Journal Of Food Microbiology 1998, 45: 43-53). To our knowledge, nothing similar to our project has ever been done. In general, the broth's composition is adapted to the metabolism of the target bacteria (see Altwegg M, Buser J, Vongraevenitz A: Stool cultures for shigella spp: improved specificity by using macconkey agar with xylose. Diagnostic Microbiology And Infectious Disease 1996, 24: 121-124) or a toxic compound, for which the target bacteria is less sensitive than most of the other bacteria, is added (see Chen H, Fraser A D E, Yamazaki H: Evaluation of the toxicity of salmonella selective media for shortening the enrichment period. International Journal Of Food Microbiology 1993, 18: 151-159). The major problem of these media is the slow growth rate or the difficulty to obtain together the high selectivity and the high growth rate.
A well known DNA amplification technique called PCR (polymerase chain reaction) is also used to detect a genetic signature of a target pathogenic bacterium in samples containing very small amount of microorganisms (see Fluit A C, Widjojoatmodjo M N, Box A T A, Torensma R, Verhoef J: Rapid detection of salmonellae in poultry with the magnetic immuno-polymerase chain-reaction assay. Applied And Environmental Microbiology 1993, 59: 1342-1346; Olsen J E, Aabo S, Hill W, Notermans S, Wernars K, Granum P E, Popovic T, Rasmussen H N, Olsvik O: Probes and polymerase chain-reaction for detection of food-borne bacterial pathogens international. Journal Of Food Microbiology 1995, 28: 1-78.). The need to selectively grow the bacteria before the detection itself is less imperative here. Nevertheless, an enrichment culture is always obligatory and the detection procedure (PCR followed by electrophoresis analysis) is time consuming and necessitates specialized technician and equipment.
An alternative method based on immunoseparation is also used (see Blackburn C D: Rapid and alternative methods for the detection of salmonellas in foods. Journal Of Applied Bacteriology 1993, 75: 199-214; Mansfield L P, Forste S J: Immunomagnetic separation as an alternative to enrichment broths for salmonella detection. Letters In Applied Microbiology 1993, 16: 122-125). Magnetic beads coated with an antibody that recognizes the target bacteria allow to specifically separate the bacteria from a pre-enriched culture. Apparently, the method is rapid but there could be some problems with the separation step which is avoided in our strategy.